ci: remove the Agents SDK downstream check (#3750)

## Summary

Remove the Agents SDK compatibility check from this repository's CI,
matching
[openai-node#2033](https://github.com/openai/openai-node/pull/2033).
Downstream compatibility is useful signal, but it belongs outside the
SDK repository, where failures can be handled without holding up
unrelated SDK changes.

Intentional type changes can create a chicken-and-egg problem: Agents
needs the SDK change before it can adapt, while SDK CI waits for Agents
to compile. Requiring a pinned downstream checkout also adds ongoing
maintenance and can keep reporting incompatibilities that Agents has
already fixed.

This removes the Agents job, its dedicated pins and build exceptions,
and the tests that only supported it. The SDK's own breaking-change
checks and dependency protections remain unchanged.

For security reasons we added pins in
https://github.com/openai/openai-python/pull/3641 but makes it very
difficult to maintain this check